In recent years, a forming method called peen forming or shot peening is in wide use in a case where a metallic plate having a large area and a complex curved surface, such as aircraft wings, is formed. As disclosed in PTL 1 and 2, according to this forming method, a metallic plate-shaped workpiece is retained and a steel ball with a diameter of approximately 0.5 mm to approximately 4 mm, which is called a shot, is projected at a high speed toward the plate-shaped workpiece to cause a strong collision, generate plastic strain in the plate-shaped workpiece, and perform curvature forming into a desired shape.
It is known that elastic stress (stress) that is applied to the plate-shaped workpiece accelerates a deformation of the plate-shaped workpiece and significantly increased formability is achieved when the shot is projected after curving and retaining the plate-shaped workpiece in an elastic deformation range thereof in advance during the peen forming. This technique is called stress peen forming. In PTL 2, as shown in FIG. 8(b) thereof, the plate-shaped workpiece is forced to be along a backing tool and is fixed with a fixing tool during the stress peen forming such that the plate-shaped workpiece is elastically deformed.
In addition, a plate-shaped workpiece may be retained in a curved state by using a clamp-shaped retaining device and a retaining device using a hydraulic jack without using a backing tool. In this case, a template (R-shaped template) that has a predetermined curvature allowing for spring back is applied to a curved surface of the plate-shaped workpiece and the predetermined curvature is set while the curvature is checked such that stress is applied by the above-described retaining devices for the plate-shape workpiece to have a curvature equal or similar to that of a completed shape after peen forming.